ABSTRACT: The Southern California Bight covers a region of about 300 km of coastline from northern Baja California to Point Conception. The present study includes ascidian species abundance data from 1994 to 2000 from 29 sites within the 12 major harborsof the Bight north of the Mexico/US border, plus 2 sites in Ensenada (Baja California, Mexico) sampled in August 2000. Nonindigenous ascidians form a major part of the fouling community biomass on floating docks in these harbors. Nine solitary and 5colonial species are present, 13 of them in San Diego Bay, the largest bay in this region. The most northerly and among the smallest bays, Santa Barbara, contains 8 nonindigenous species. This study reports additional surveys to our 1998 paper (Lambert &Lambert 1998, Mar Biol 130:675-688) for fall 1997, spring 1998 and summer 2000, documents the newly arrived Botrylloides perspicuum, and corrects the records of 2 species. A main focus is the listing and abundance of species at each survey sitewithin each major harbor and an attempt to analyze the differences between sites and between seasons within a single harbor with reference to variations in water movement, temperature, salinity, pollution factors and competition. Several species appearedfor the first time in southern California during 1994 to 1997, apparently correlated with the influx of warmer water during the El Niño of that time. Subsequent surveys show that all of these species have persisted, several have dramatically increased inabundance and distribution, and an additional species has successfully invaded.